tty/n_hdlc: Fix struct n_hdlc kernel-doc warnings

tty/n_hdlc: Fix struct n_hdlc kernel-doc warnings

Add missing descriptions for write_work and tty_for_write_work
members in struct n_hdlc.

This fixes the following warnings:
drivers/tty/n_hdlc.c: warning: Function parameter or member
  			       'write_work' not described in 'n_hdlc'
drivers/tty/n_hdlc.c: warning: Function parameter or member
  			       'tty_for_write_work' not described in 'n_hdlc'

Signed-off-by: Bartlomiej Kubik <kubik.bartlomiej@gmail.com>
Link: https://patch.msgid.link/20251202063748.1210359-1-kubik.bartlomiej@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>